Engines may be configured with an exhaust heat recovery system for recovering heat of exhaust gas generated at an internal combustion engine. The heat recovered at an exhaust gas heat exchanger may be utilized for functions such as heating the cylinder head, and warming the vehicle cabin, thereby improving engine, and fuel efficiency. Exhaust heat may be utilized for expediting exhaust catalyst light-off and for maintaining catalyst temperature within a desired range for optimal catalyst performance. Cooled exhaust gas may be recirculated from upstream of exhaust catalysts to the intake manifold and used to reduce fuel consumption, and exhaust NOx emissions. An EGR cooler may be coupled to an EGR delivery system to bring down the temperature of recirculated exhaust gas before it is delivered to the intake manifold.
Various approaches are provided for exhaust heat recovery and EGR cooling. In one example, as shown in US 20140196454, Ulrey et al. discloses an engine system with a post-catalyst EGR cooler that may be opportunistically used to recover exhaust heat. During cold-start conditions, an exhaust throttle valve may be closed to direct exhaust through the EGR cooler wherein heat from the exhaust may be transferred to a coolant circulating through the EGR cooler. The warmed coolant may then be circulated through the engine to increase engine temperature. During such cold-start conditions, the EGR valve may be maintained in a closed position, and after flowing through the EGR cooler, the exhaust may return to the main exhaust passage via a bypass passage.
However, the inventors herein have recognized potential disadvantages with the above approach. As one example, it may not be possible to recover exhaust heat and simultaneously provide EGR. In the system shown by Ulrey et al., during any engine operating condition, exhaust sourced from a single location, downstream of the exhaust catalysts, may either be used for exhaust heat recovery or be delivered as EGR. Also, exhaust cannot be sourced from upstream of the catalysts for recirculation to the intake manifold. Further, due to the configuration of coolant flow, irrespective of the cooling needs, the same degree of cooling is provided during both EGR cooling and exhaust heat recovery. The coolant flow configuration may also result in overheating of coolant flowing through the heat exchanger, causing degradation of the coolant system.
The inventors herein have identified an approach by which the issues described above may be at least partly addressed. One example method comprises: transferring heat from a first portion of exhaust flowing through an exhaust gas recirculation (EGR) passage to coolant n a first branch of a heat exchanger, and transferring heat from a second portion of exhaust flowing through an exhaust bypass to coolant in a second branch of the heat exchanger, a direction of coolant floe through the first and the second branch selected based on coolant temperature. In this way, two portions of exhaust sourced from different locations in the main exhaust passage may be cooled simultaneously via a single split heat exchanger, while the direction of coolant flow through the heat exchanger is dynamically adjusted based on coolant temperature.
In one example, a boosted engine system may be configured with a single split heat exchanger configured for concurrent EGR cooling and exhaust heat recovery. A first exhaust bypass passage may couple a main exhaust passage from upstream of an exhaust turbine to an EGR delivery passage, the EGR delivery passage recirculating exhaust gas to an intake passage. A first branch of the heat exchanger may be coupled to the EGR delivery passage while a second branch of the heat exchanger is coupled to a second exhaust bypass passage, downstream of the first bypass passage and downstream of one or more exhaust catalysts. During cold start conditions, tailpipe emissions may be lowered by restricting (and retaining) exhaust within the exhaust manifold and first bypass passage for a threshold duration to increase exhaust heat transfer. After the threshold duration has elapsed, exhaust may be routed to the catalysts via the first bypass passage, bypassing the turbine. After flowing through the catalysts, exhaust from the main exhaust passage (from downstream of the exhaust catalysts) may be routed to the tailpipe via the second exhaust bypass passage to enable exhaust heat recovery at the heat exchanger. A diverter valve coupled to the main exhaust passage may be adjusted to regulate the portion of exhaust routed via the second bypass passage. Exhaust heat may be recovered via a coolant circulating through the heat exchanger. After catalyst light-off, a first portion of exhaust may be routed to the intake manifold from upstream or downstream of the turbine in the main exhaust passage via the first bypass passage and the EGR delivery passage based on engine load. A second portion of exhaust may be routed via the second bypass passage for exhaust heat recovery. Coolant may be routed simultaneously through each of the first and second branches of the heat exchanger to concurrently cool EGR flowing through the EGR passage and extract exhaust heat from exhaust flowing through the second bypass passage. Warmed coolant may then be used for engine and cabin heating, as required. A direction of coolant flow through the heat exchanger may be adjusted based on the exhaust cooling demand and coolant temperature. For example, when the exhaust cooling demand is higher (such as when the exhaust temperature is higher), a higher degree of heat transfer may be achieved by flowing coolant through each of the first and second branch of the heat exchanger in a direction opposite to the direction of exhaust flow through each of the EGR passage and the second bypass passage. In comparison, when the exhaust cooling demand is lower (such as when the exhaust temperature is lower), a lower degree of heat transfer may be achieved by flowing coolant through each of the first and second branch of the heat exchanger in the same direction as exhaust flow through each of the EGR passage and the second bypass passage. In alternate examples, coolant flow may be provided opposite to the direction of exhaust flow when the coolant temperature is lower (wherein the coolant can take up more exhaust heat), and coolant flow may be provided in the same direction as exhaust flow when the coolant temperature is higher (wherein the coolant can take up less exhaust heat).
In this way, by adjusting the direction of coolant flow through each branch of the heat exchanger, a desired level of exhaust heat transfer may be provided based on each of an EGR cooling demand and an engine heating demand. By opportunistically adjusting a direction of coolant flow through the heat exchanger based on coolant temperature, overheating of coolant may be averted. The technical effect of containing exhaust within a section of the exhaust manifold during cold start conditions is that cold-start emissions may be reduced while enhancing exhaust heat recovery in that section of the exhaust manifold. By routing hot exhaust directly to the catalysts bypassing the turbine, attainment of catalyst light-off temperatures may be expedited. By providing the functions of an EGR cooler and an exhaust gas heat exchanger via a single split heat exchanger, cost and component reduction benefits are achieved without limiting the functionality or capability of either system, and while enabling EGR to be sourced from both upstream and downstream of an exhaust turbine. By recovering heat from exhaust downstream of the catalysts, attainment of light-off temperature of catalysts may not be affected and temperature of the catalysts may be maintained above their light-off temperatures. Overall, by simultaneously providing EGR and exhaust heat recovery capabilities, fuel efficiency may be improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.